1. Field of the Invention
The present invention relates to a coolant having a high cooling capability for use in cryogenic devices. The coolant is particularly suitable as a cooling medium or a heat exchange medium when semiconductor devices such as those which can exhibit a high speed switching at a cryogenic temperature of about 123 K or less, such as 77.3 K, i.e,, a boiling point of liquid nitrogen (LN.sub.2), or other devices such as those using a superconducting material are cooled to a working temperature thereof. Note, any cryogenic temperature around the boiling point of liquid nitrogen is referred to herein as a liquid nitrogen temperature The present invention also relates to a method for the production of the coolant, a method of cooling an article to the cryogenic temperature, and a cryogenic device using the coolant.
As is well known in the art, the mobility of a carrier in the semiconductor can be increased by reduction of a temperature to which the semiconductor is subjected, and based on this temperature characteristic, high speed switching devices which are operable at the liquid nitrogen temperature, such as high electron mobility transistors (HEMTs) or complementary metal oxide semiconductor (CMOS) devices, have been developed. For example, M. Abe, T. Mimura and M. Kobayashi, "Ultra-High-speed HEMT LSI Technology", FUJITSU Sci. Tech. J., 24, 4, PP. 271-283 (December 1988) teach that HEMTs developed by Fujitsu Limited have achieved a 5.8 ns switching speed, and T. Vacca, D. Resnick, D. Frankel, R. Bach, J. Kreilich and D. Charlson, "A Cryogenically Cooled VLSI Super-computer", VLSI SYSTEMS DESIGN, June 1987, pp. 80-84 teach that the first cryogenic computer, using a 77K CMOS was shipped last year. To operate these devices at the liquid nitrogen temperature with satisfactory results, it is necessary to use a high power coolant or cooling medium which can constantly maintain a desired cryogenic temperature such as the liquid nitrogen temperature, and also enables an effective dissipation of the heat generated in the devices during operation thereof to be obtained.
2. Description of the Related Art
Hitherto, to cool the semiconductor devices and other devices to a cryogenic temperature of about 123K or less and to maintain such a temperature, it has been proposed to arrange a heat generating part of the device in contact with a cold head of a refrigerator to dissipate the heat generated from the device through a heat conduction, or to immerse the device in a cryogenic fluid such as a liquefied gas, for example, a liquid nitrogen, to dissipate the heat from the device through a boiling heat transfer.
More particularly, the latter cooling method based on the boiling heat transfer can be advantageously used when the devices to be cooled have a large heat generation density and a complex structure or configuration. For example, this cooling method can be applied to the cooling of central processing units (CPUs) of a computer. Further, although a variety of liquefied gases are available, the liquefied gas usable as the cryogenic fluid in this cooling method is limited in view of the toxicity and reactivity of such gases. Suitable liquefied gases having a stable and simple composition are a liquefied nitrogen, as described above, and a liquefied helium. The liquefied nitrogen is obtained by liquefying air and separating the resulting fluid.
Use of the liquefied nitrogen as the cryogenic fluid is described in, for example, F. H. Gaensslen, V. L. Rideout, E. J. Walker and J. J. Walker, "Very Small MOSFET's for Low-Temperature Operation", IEEE TRANSACTIONS ON ELECTRON DEVICES, MARCH 1977, in which FET's are cooled by direct immersion in an open pool of the liquefied nitrogen. Further, the use of the liquefied fluorocarbons as the cryogenic fluid is described in, for example, "Cooling a Superfast Computer", ELECTRONIC PACKAGING & PRODUCTION, JULY 1986, in which the entire computer is immersed in a non-conductive circular fluorocarbon liquid. Furthermore, the use of a mixture of liquefied nitrogen and liquefied fluorocarbon (CF.sub.4) as the cryogenic fluid is described in, for example, T. Amano and M. Nagao, "Boiling Heat transfer Characteristics of Mixed Coolant", 1988 Cryogenic Society of Japan Spring Meeting, C1-4, May 1988, in which a superconducting material having a high Tc is directly immersed in a mixed coolant consisting of the liquefied nitrogen and liquefied CF.sub.4 in an open can.
The cooling method taught by Amano et al is illustrated in FIG. 1. In order to ascertain an effectiveness of the mixed coolant when cooling the high Tc superconducting material, the illustrated experimental apparatus in which a Pt wire 1 is dispersed in a mixed coolant 2 in an open vacuum can 3 was used. A thermocoupler 4 as a temperature sensor connected with a recorder 8 is also disposed in the mixed coolant 2. In the apparatus, 5 is a balance-scale used to determine a change of the weight of the coolant 2, 6 is a DC supply, and 7 is a resistor. Surprisingly, the experiments using this apparatus showed that the mixed coolant of the liquefied nitrogen and 8% by mole of the liquefied CF.sub.4 can increase the cooling capability to about twice that of the cooling capability obtained when the liquefied nitrogen is used alone. Although this paper does not teach the use of the mixed coolant (LN.sub.2 plus liquefied CF.sub.4) when cooling semiconductor devices and related devices, it is understood that such a use will be accompanied by many drawback, for example:
(1) since an open system is applied, an atmospheric air or a moisture in the atmosphere can be incorporated into and frozen in the mixed coolant and;
(2) since the coolant is boiled and vaporized, the gas generated can cause atmospheric pollution and an additional coolant must be supplied to the open can to compensate for a vaporization loss of the coolant.
In order to avoid the drawbacks (1) and (2), use of the mixed coolant in the closed system has been considered, but since the mixed coolant must contain a higher amount of the liquefied CF.sub.4, i.e., as high as 8 to 20% by moles, a composition of the mixed coolant will be changed upon liquefaction of the vaporized coolant, thereby causing variations in the cooling capability of the coolant. Further, in the vaporized coolant, the high boiling components thereof tend to be condensed and separated in a liquefied or refrigerator used to again liquefy the vaporized coolant, because of a high concentration of the high boiling components. The separated components will cause clogging of the pipes, a stoppage of the apparatus and other unavoidable drawbacks. Furthermore, not all fluorocarbons except for CF.sub.4, which are considered to be an appropriate admixture to the liquefied nitrogen, provide a solution after they are directly admixed with the liquefied nitrogen in accordance with conventional mixing methods; in practice, they are solidified and precipitated in a mixing apparatus. Note, the inventors also tried to use a mixed coolant consisting of the liquefied nitrogen and the liquefied argon or the liquefied krypton, but could not obtain satisfactory cooling effects.
On the other hand, to obtain an increased cooling capability, use of a combination of a cryostat or closed cryogenic container and a liquefier (more particularly, a refrigerator thereof), a heat exchanger pipe of which liquefier is disposed in an upper inner portion of the cryostat. An article to be cooled is immersed in a coolant or cooling medium in the cryostat, and the vaporized coolant is liquefied as a result of heat exchange between a vapor of the coolant and a heat exchange medium circulated through the heat exchanger pipe. In this cryogenic cooling system, a liquefied nitrogen is generally used as the coolant, and a helium gas is used as the heat exchange medium. This system, however, suffers from the following drawbacks:
(1) to avoid undesirable effects on the article to be cooled due to vibration of the liquefier, the liquefier must be disposed far from the cryostat;
(2) since the helium gas is used as the heat exchange medium, the resulting cooling capability is not satisfactorily high, as apparently, the heat exchange is made based on a gas-gas contact and;
(3) to increase a cooling capability of the gas-gas contact system, it is possible to increase a diameter of the heat exchanger pipe used. This, however, will result in an increase of the power and equipment area of the apparatus and thus increase operation costs.
Therefore, it is desired to provide an improved coolant or cooling medium having a high cooling capability for use in cryogenic devices, and not having the drawbacks of the prior art coolants.